The invention relates to communications networks and, more particularly, to switches and routers for communications networks that employ the TCP/IP Internet protocol suite.
Communications networks, such as local area network (LAN), wide area networks (WAN), and the Internet, support an increasing number services. For example, voice, facsimile, and video services have been added to traditional packet-based services. Many TCP/IP applications on such multi-service networks rely upon the IP layer for transport. Although the reliability and speed of such networks was adequate for traditional packet-based traffic, the delivery of voice, facsimile and video information typically requires a higher level of reliability and, in the event of a failure, more rapid recovery from a failure than is generally available from a conventional TCP/IP communications network. Optimized for spare link efficiency and topological flexibility, such conventional IP networks rely upon best-effort packet delivery. Improved reliability is required for TCP/IP applications that rely on the IP layer for transport. However, packet level granularity, connectionless based transport, and hop-by-hop routing all conspire to expand the IP restoration time to something on the order of tens of seconds or even minutes. An interruption of this length can prove very costly in any of the new applications. That is, an interruption of only half a second during a telephone conversation can be very annoying, the seconds-long loss of a video signal during a football game may obscure a critical touchdown (or non-touchdown called as a touchdown), and the loss of seconds from a facsimile signal could require re-sending the facsimile. Such performance limitations may preclude the acceptance TCP/IP networks for the delivery of various such applications.
Compared to connection-oriented protection approaches, such as SONET, IP restoration is slow and unpredictable. Typically, IP compliant communications networks include a plurality of paths between IP routers or switches (for the sake of clarity, the term xe2x80x9croutersxe2x80x9d will be used hereinafter to describe both routers and switches). The routers select among the various paths to form a circuit that permits the delivery of signals from an entry point to an exit point within the network. When one of the selected paths fails, the failure must be detected, the fault information must be propagated so that all the affected circuits may be reconfigured, and, finally, the appropriate response, re-routing of the circuits, must be computed and effected. This three-step process may require minutes to complete. Although upper layer protocols and applications, such as TCP retransmission, sufficiently addresses reliability problems for conventional packet-based applications, IP restoration is too slow and unpredictable for the voice, video, fax, or virtual private network applications which otherwise might employ IP networks as a multi-service backbone.
A TCP/IP communications network that provides rapid restoration, thereby permitting the use of such networks for the reliable, readily restored transmission of voice, video, fax, or virtual private network signals would therefore be highly desirable.
A router in accordance with the principles of the present invention employs explicit routing protocols to establish a plurality of explicitly routed paths between source and sink routers. The sink router selects one of these explicitly routed paths as a primary path and communicates along that path. Upon a failure in a path selected as a primary path, a secondary path is instantaneously selected as the new primary path. Since the new route has already been established, there is no need to compute the path at this at this time-sensitive juncture. One of the new routers may employ physical level maintenance information, such as loss of signal (LOS) or loss of pointer (LOP), for example, to detect such path failures. In another aspect of the invention, the new router may employ provisioned flow information in order to propagate failure information.
A router in accordance with the principles of the present invention may be employed to establish one or more circuit paths among a plurality of routers. Rather than establishing a hop-to-hop path in order to permit the transmission of signals along a circuit, a router in accordance with the principles of the invention operates as an explicitly routed line switched router (ERLSP) to establish a plurality of paths from a source (entry) router to a sink (destination) router. The paths are provisioned at the source router, through a network management system, for example, which may, in accordance with the principles of the present invention, ensure that the paths are disjoint. All of the new routers between the source and sink routers operate to establish the plurality of paths. In the event of a path failure, the sink router selects an operational one of the pre-established paths. Additionally, in order to accommodate a failure in the newly selected path, the sink and source nodes may establish another path back to the source router to maintain the desired redundancy and the secondary (and ternary, etc.) path(s) may also be monitored for failure so that they may be replaced in the event of their failure.
That is, a relatively simple network implementation in accordance with the principles of the invention may entail the establishment of a primary path and secondary, or backup, path for each router in the network. This concept may be extended to include a plurality of primary paths, with one or more secondary paths for each of the primary paths. An operations control center may provision each of the routers with the number and type (that is, primary or secondary) of each path. Multiple secondary paths would accommodate multiple failures, but at the price of more complexity in the provisioning and in the operation of the protection scheme. Regardless of the number of primary and secondary paths, in accordance with the principles of the invention a failure may be detected through physical layer indicators in a primary path. In response to the physical layer failure indication, the failure is propagated and the exit router selects an alternative, previously established, path for immediate use.
A router in accordance with the principles of the invention may employ physical level maintenance information for failure detection. In a network that provides maintenance information, such as SONET fault indicators like loss of signal (LOS), loss of pointer (LOP), a router in accordance with the principles of the present invention may employ such indicators to determine when a routing path has failed. Because the physical level maintenance information indicating a failure is typically available to routers much more rapidly than conventional path failure indications, a communications system which employs one of the new routers may be alerted to path failures more rapidly than conventional routers.
Additionally, a router in accordance with the principles of the invention may employ provisioned flow information in order to propagate failure information. By propagating the failure information in this manner, rather than by a conventional approach, which encounters hop-by-hop routing delays, a communications system may propagate failure information more quickly than conventional routers would allow.
The IP flow ring protection switching mechanism provided by a router in accordance with the principles of the present invention can provide restoration in substantially less time than that required for conventional IP restoration mechanisms. Additionally, because a router in accordance with the principles of the present invention operates at the physical layer, it is independent of link layer protocols. Consequently, the router, and network systems which employ it, automatically support asynchronous transfer mode (ATM), frame relay (FR) and point-to-point (PPP) protocols.
A router in accordance with the principles of the present invention, and IP flow rings employing such a router, employ explicit routing algorithms to establish a plurality of explicitly routed circuit paths. The sink router chooses one of these paths as the primary path and communicates along this primary path unless the primary path fails. If the primary path fails, the sink router switches to communications over the secondary path. In those systems where physical or link level maintenance information is available, all the routers along the explicitly routed paths may monitor this information to quickly detect any path failures. For example, in a SONET-based system the routers may employ SONET fault indicators to detect path failures. If such a failure is detected, the router that first detects the failure propagates this information to the source and sink routers. The failure information may be propagated, for example, through provisioned flow information. When the source and sink routers are alerted to the path failure, the sink router switches to the secondary path for communications. The source router may then establish another explicitly routed communications path to act as a new secondary path.
The routers along the secondary path may also monitor the path, and propagate failure information, as described above, so that the source and sink routers may establish another secondary path in the event of a secondary path failure.